Vinyl chloride polymers possess poor flow properties, low stability under dynamic processing conditions, are difficult to shape, and present difficulties when attempts are made to flow or melt them.
In converting vinyl chloride polymers into molded articles, the polymer powder is generally mixed with various additives, e.g. auxiliary processing media, such as stabilizers, lubricants (mold release agents), pigments, and fillers, depending on the properties to be imparted to the composition. The powder (compound) is heated to melt it into a homogeneous or gelled mass. For reasons of economy alone, it is desirable to keep such a heat treatment as brief as possible, i.e. it is desirable to process a polymeric material that melts or gels rapidly. Thus, the aim is for vinyl chloride polymers to be processable at high speed in order to attain high output rates with high-speed processing machines. The high-speed processing must not impair properties such as mechanical strength, transparency, and surface characteristics. Since unmodified polyvinyl chloride is not rapidly processable, it is necessary to add auxiliary processing media to the polyvinyl chloride composition.
Processes for improving the processing properties of polyvinyl chloride are conventional. For example, it is known to add, to the finished polyvinyl chloride, copolymers of vinyl chloride with other monomers, other polymers, e.g. acrylonitrile-styrene copolymers, methyl methacrylate-styrene copolymers, and also graft copolymers of a quite specific structure (DAS's [German Published Applications] Nos. 2,414,561 and 1,952,839).
However, blending polyvinyl chloride with other polymers requires an additional working step for the processor and necessitates keeping a separate store of modifier.
The mentioned graft copolymers of complicated structure (DAS's Nos. 2,414,561; 2,363,564; and 1,952,839), which must be blended with the polyvinyl chloride composition prior to the desired processing, can be produced only with great technical expenditure.
Also, the incorporation of aqueous additive mixture dispersions (DOS [German Unexamined Laid-Open Application] No. 2,709,404) does not provide the processor with any marked advance in the art, since the water contained in the dispersion must be vaporized out of the molding composition later, with considerable extra expenditure.
The polymerization of vinyl chloride up to a conversion of 60-80%, subsequent removal of the unreacted vinyl chloride, addition of methyl methacrylate, and subsequent methacrylate polymerization are technically unsatisfactory (DOS No. 2,422,688). Large amounts of return (unreacted) VC are obtained; unreacted methyl methacrylate must be removed from the cycle. Moreover, the modifier content is very high (35-65%). The products are blended for processing with unmodified polyvinyl chloride, which necessitates an additional operating step with the extra expenditure connected therewith.
The subsequent metered feeding of a comonomer (DAS No. 1,795,390) at a specific conversion requires an accurate determination of the conversion as well as expensive regulating and metering devices. Moreover, it means that, for example, for each percent of introduced vinyl acetate, the softening point of the homopolymer/copolymer mixture drops by 0.5.degree. C. (DOS No. 2,606,934). Although vinyl chloride-ethylene copolymers possess improved processability, just as vinyl chloride-propene copolymers, this is also attained at the cost of a lowered thermal deformation temperature. Additionally, the control of the reaction is difficult, since monomeric propene is a degrading chain transfer agent and slows down the reaction velocity.
Consequently, the production of modified PVC which can be processed at high speeds into sheet stock, for example on calenders, is difficult from the viewpoints of copolymer production, homo-copolymer mixtures required, and the required separate production of graft copolymers; and/or it represents additional expenditure in apparatus and time for the processor.